The present application is based on French patent application 0009640, filed on Jul. 20, 2000, which is hereby incorporated by reference in its entirety.
Field of the Invention
The invention relates to a hollow composite body and its manufacturing process, the hollow body consisting of an internal layer of thermoplastic, a composite interlayer made of a thermoplastic in which reinforcing fibers are embedded, and an external layer made of a thermoplastic.
Cylindrical hollow bodies of this type, intended to contain pressurized fluids for example, are known from Patent Application WO 00/24566. Melting of the internal layer with the interlayer is obtained by a manufacturing process in which a thermoplastic tape having embedded continuous glass fibers is wound around a primary structure, or xe2x80x9clinerxe2x80x9d (which term will be employed in the rest of the description). This is done by heating the surface of the liner which is to be covered with the tape, in a zone located immediately adjacent the zone where the tape comes into contact with the liner, to a temperature above the melting point of the thermoplastic, then applying local pressure to the surface of the liner already covered with the tape in a zone located immediately adjacent the heating zone. This process makes it possible to completely fuse the thermoplastic of the liner and that of the composite tape and to remove the air between the layers of tape covering the liner. Finally, in order to coat the cylindrical body thus obtained with the external protective thermoplastic finishing layer, the process continues with a coextrusion step.
This process proves to be particularly advantageous in the case of elongate hollow bodies of cylindrical shape but is much more difficult, or sometimes impossible, to use for more complex shapes, especially when coextruding the external thermoplastic layer.
Patent Application WO 00/15415 proposes the manufacture of a composite pressurized tank of non-cylindrical shape by covering a thermoplastic liner with a layer of a glass fiber based composite by filament winding, placing this semifinished product or preform (this latter term being employed hereafter) in a mold, having beforehand introduced an inflatable bag thereinto, and in raising the temperature of the assembly while applying pressure in the liner by inflating the bag. This makes it possible to consolidate the composite layer with the liner.
For a tank of this type, requiring an external functional thermoplastic covering around the composite layer, a preform composed of the liner and the composite layer is surrounded with a thermoplastic film before it is inserted into the mould. The three constituents, namely the liner, the composite layer and the film, are then consolidated by heating the mold and inflating the bag.
This technique for producing the external layer of a complex shape therefore requires the product to be surrounded with a thermoplastic film. However, this operation can prove to be difficult and lengthy to implement when the complex structure has contours of variable shapes, the operator having to take particular care to suitably coat the details of its contours. The cost of such a solution is also to be taken into consideration since the use of such a thermoplastic film and the time spent in applying it consequently increase the manufacturing cost of the product.
It is therefore an object of the invention to manufacture a hollow composite body comprising an external covering layer made of a thermoplastic in a simpler and less expensive manner.
According to the invention, a hollow body includes an external layer formed from the thermoplastic of the interlayer and possibly of the internal layer, which thermoplastic has migrated beyond the interlayer during manufacture of the hollow body. This hollow body thus constitutes a homogeneous product since the continuity of the thermoplastic of the liner up to the external layer ensures perfect consolidation of the components. This is not necessarily guaranteed when the constituent layers are, as in the prior art, added successively one after the other and then consolidated.
Furthermore, this solution proves to be quicker to implement than that of the prior art by dispensing with one step in the process, since it is not necessary to apply a thermoplastic film.
According to one characteristic of the invention, the hollow body, before migration, comprises the liner and the composite interlayer covering the said liner, the migration of the thermoplastic taking place after pressurization of the internal wall of the liner and heating of the liner covered with the interlayer.
Surprisingly, the present inventors have found that the material of the interlayer migrates beyond the interlayer during its consolidation with the liner, and that the means employed for the consolidation can be tailored in order to also make the thermoplastic of the liner migrate, so as to increase the thickness of the external layer.
Advantageously, the thermoplastic of the composite interlayer is identical to that of the internal layer. Preferably, the thermoplastic is a polyolefin, for example polypropylene or polyethylene, or else a thermoplastic polyester such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), and the reinforcing fibers are glass fibers.
The process for manufacturing this hollow composite body comprises the following steps: furnishing the external surface of the liner with a composite so as to constitute a preform, the composite consisting of a thermoplastic mixed with reinforcing fibers; introducing the preform into a mold, an inflatable bladder having been inserted into the preform; and heating the preform and inflating the inside of the bladder in order to apply pressure against the internal wall of the preform, wherein a space between the external wall of the preform and the internal wall of the mold is tailored to the thickness of the external layer of the hollow body to be obtained, so as to allow migration of the thermoplastic of the interlayer, and possibly of the liner, beyond the interlayer during the heating and pressure application step.
According to one further characteristic of the invention, the composite is deposited by winding a tape which is delivered by an applicator from a multiplicity of wound packages of composite yarn advantageously consisting of co-mingled glass and thermoplastic filaments.
According to another further characteristic of the invention, the pressure applied to the inside of the bladder is between 1 and 10 bar, while the temperature to which the preform is heated is at least equal to the highest melting point of the thermoplastics present in the preform.